202 CHEMICAL ENGINEERING of instances, bub it is the general conclusion that unequal wear cannot be entirely avoided by any means so far discovered. The roll is highly efficient under some conditions, particularly if properly designed as to the relation of the roll diameter to the size of feed, if operated at proper speed and given proper care. With abrasive feed, however, it is almost impossible to prevent unequal wear and other machines are often considered more advisable for such service. The theoretical capacity of rolls may easily be calculated by computing the cubical contents of the ribbon passing through the machine, the data necessary includes the speed of the roll, the width of the roll face and the distance apart of the two cylinders. This will give the cubical volume that will pass over the rolls in a given time. In practice, due to the unequal character of the feed, not more than one-quarter of this cubical volume should be taken as the actual capacity of the machine. As to power, the accompanying formulae show the requirements for crushing rolls of all kinds. The power to drive rolls at normal speed, without load, varies for different sizes, speeds and makes of machines. An approximate idea being obtained from the following formula.1 Pi = 0.0835(D + TF) When, D = Diameter of rolls in inches, and W = Face of rolls in inches. The total power required to drive rolls, loaded is: P = (p X M) + Pi When, P = Total power, p = Horsepower per cubic foot per hour (from Fig. 6, p. 201), PI = Horsepower required to drive rolls without load, and M = Capacity of rolls in cubic feet per hour. FEED SIZE FOR 32-DEG. NIP ANGLE ON ROLLS Roil, Space between rolls, inches diam- /4 /a H 7a S4- /8 0 eter, inches Maximum size of feed, inches 36 2.23 2.10 1.96 1.84 1.71 1.57 1.45 30 1.99 1.86 1.73 1.60 1.47 1.34 1.21 26 1.83 1.70 1.56 1.44 1.31 1.17 1.05 24 1.74 1.61 1.48 1.36 1.22 1.10 0.96 20 1.58 1.46 1.32 1.20 1.06 0.94 0.80 16 1.42 1.29 1.16 1.03 0.90 0.77 0.64 9 1.14 1.01 0.88 0.75 0.62 0.49 0.36 In addition to the types already mentioned for crushing, there are used in various instances other devices such as, edge runners of the Chilean mill type, ball mills and various specially designed machines for accomplishing this work. The Chilean mill is a well-known rock reduction machine, but since its field is usually in fine reduction, it will be considered more exhaustively under the head of grinding. For crushing purposes, Chilean mills with comparatively slow move- 1 These formulas from "Electric Motors in the Cement Industry" by R. B. WILLIAMSON, Am. Inat. Blec. Eng., compiled from data gathered by the Committee on Industrial and Domestic Power.